We are analyzing the interactions between virus glycoproteins and their receptors for several enveloped viruses important for human health. Our goals are to define mechanisms of virus fusion and entry, elucidate mechanisms of virus tropism, identify cellular receptors, develop strategies to elicit and detect neutralizing antibodies, and develop novel therapeutic and preventative approaches. 1) Human herpesvirus 6 (HHV-6) is the etiologic agent of exanthema subitum and causes opportunistic infections in immunocompromised patients; it has also been implicated in multiple sclerosis and in the progression of AIDS. We previously discovered that the two major HHV-6 subgroups (A and B) use human CD46 as a cellular receptor. During the past year, we performed molecular characterization of the HHV-6 interaction with CD46. One study is directed an characterizing the regions of CD46 critical for HHV-6 entry. Fusion assays with chimeric and truncated forms of CD46 revealed that the determinants of this molecule critical for HHV-6 are distinct from those critical for measles virus (which also uses CD46 as an entry receptor). Another study seeks to identify the specific HHV-6l glycoprotein(s) involved in binding to CD46. Co-immunoprecipitation experiments suggest the probable involvement of the viral gH glycoprotein. Additional collaborative experiments have been initiated to study HHV-6 infection in a CD46 transgenic mouse model. 2) Hepatitis C virus is a major human pathogen associated with chronic liver disease and hepatocellular carcinoma. Research on this virus is severely hampered by the absence of a suitable cell culture system in which to propagate the virus and study its life cycle. We have expressed molecular chimeric variants of both the E1 and E2 glycoproteins that enable surface expression of these glycoproteins. The E1/E2 expressing cells were shown to fuse with hepatocyte cell lines in a low pH-induced manner. We are optimizing this system for analysis of neutralizing antibodies, and for screening cDNA libraries for the cellular receptor(s).